1. Field of the Invention
This invention relates to radiation curable compositions, more particularly radiation curable inks and inkjet inks and their use in inkjet printing processes and ink sets.
2. Description of the Related Art
In inkjet printing, tiny drops of ink fluid are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a mechanism for ejecting the drops image-wise. Printing is accomplished by moving a print head across the ink-receiver or vice versa or both.
When jetting the inkjet ink onto an ink-receiver, the ink typically includes a liquid vehicle and one or more solids, such as dyes or pigments and polymers. Ink compositions can be roughly divided in:                water-based, the drying mechanism involving absorption, penetration and evaporation;        solvent-based, the drying primarily involving evaporation;        oil-based, the drying involving absorption and penetration;        hot melt or phase change, in which the ink is liquid at the ejection temperature but solid at room temperature and wherein drying is replaced by solidification; and        UV-curable, in which drying is replaced by polymerization.        
It should be clear that the first three types of ink compositions are more suitable for an absorbing receiving medium, whereas hot melt inks and UV-curable inks can also be printed on non-absorbing ink-receivers. Due to thermal requirements posed by hot melt inks on the substrates, especially radiation curable inks have gained the interest of the packaging industry.
In general, UV curable inks contain colorants, monomers, photoinitiators and polymerization synergists. Preferred types of synergists are tertiary amines which are admixed to acrylate based radiation curable inks for two main reasons:                i) to reduce air inhibition and thereby increase cure speed of inks including Norrish I type photoinitiators; and        ii) to initiate the so called Norrish II type photopolymerisation, wherein a Norrish II type photoinitiator, e.g. a benzophenone type, abstracts a hydrogen from the amine forming radicals that promote radical polymerisation of acrylate monomers.        
Migrateable residues in coatings used for packaging of foodstuffs present a health risk and consequently they should be kept to an absolute minimum. Known measures to reduce extractables of the photo-initiating system from cured ink layers include the use of polymeric or co-polymerizable photoinitiators and synergists instead of the usual low molecular weight compounds. However, in many cases it was reported that the curing speed was significantly reduced and this was believed to be due to the fact of reduced mobility of the photoinitiator or synergist.
US 2006014848 (AGFA) discloses radiation curable inkjet inks including a polymeric co-initiator including a dendritic polymer core with at least one co-initiating functional group as an end group. Aliphatic amines and aromatic amines are included as suitable co-initiating functional groups. The dendritic polymeric architecture allows to obtain low extractables and at the same time minimizes the increase in viscosity of the ink. No polymerizable amine synergists are disclosed.
U.S. Pat. No. 4,070,262 (MOBIL OIL) discloses a UV curable coating composition wherein the adhesion of a UV cured film is improved by replacing all or part of a non-polymerizable tertiary amine with dimethylaminoethyl acrylate. The application is silent on the suitability of the coating compositions for food packaging applications.
US 2005203199 (IVOCLAR VIVADENT) discloses a dental paste including a radically polymerizable organic binder, at least one initiator for the radical polymerization and at least one accelerator for the radical polymerization, wherein both initiator and accelerator each have at least one radically polymerizable group. Suitable polymerizable amine accelerators include 4-dimethylaminobenzoic acid derivatives among others. A dental formulation is typically a high viscous composition not imposing high demands towards colloid stability in contrast to inkjet inks where the colloid stability of fine particle dispersions is of vital importance for the application. The application is silent on low viscous liquid curable compositions.
Ink jet inks have to meet very high demands on shelf life in combination with a low viscosity. This requires a dedicated design of all components of the ink jet ink to guarantee a good shelf life (storage stability) in a low viscous formulation. Food compliant inks further must have a very low amount of extractables.